Ferroelectric Liquid Crystal Film Research Articles

Photochemical switching of polarization in ferroelectric liquid-crystal films

LIQUID crystals have been used extensively as active media in display devices such as full-colour television screens. These devices are generally based on changes in the arrangement of the liquidcrystal molecules induced by electric fields, which change their optical properties1. Ferroelectric liquid crystals2,3 exhibit spontaneous polarization and therefore show a faster response to changes in the applied field. Switching of this field causes a reversal in the direction of polarization2–;5. Here we report polarization switching in ferroelectric liquid crystals driven by a photochemical process. The liquid-crystal films are doped with a photochromic compound which undergoes trans–cis isomerization on irradiation. Photoisomerization induces a change in the switching potential of the host liquid-crystal film, and thereby causes switching at the irradiated sites. The process is fast, stable, reversible and repeatable, and should be exploitable in device applications.

Boundary layer elasto-optic switching in ferroelectric liquid crystals

Utilizing a simple new configuration, one rigid surface and the other exposed directly to the gas flow, elasto-optic switching in thin (∼2 μm) ferroelectric liquid crystal films is observed when a gas flow is impingement in the plane of smectic layers. The elasto-optic response caused by the shear stress is large, fast, and reversible.

A novel boundary layer sensor utilizing domain switching in ferroelectric liquid crystals

A new sensor for optical detection of shear stress field induced by air or gas flow on a rigid surface is reported. The sensor uses the novel effects of shear induced optical switching in ferroelectric liquid crystals. The principle of operation of the sensor is described and a theoretical model for the optical response to shear stress is given. Director dynamics and system-free energy considerations predict a system response time ≊10 ns for an applied shear stress step of ≊700 Torr. A thin (≊1 μm) ferroelectric liquid crystal film coated on a flat glass model surface is exposed to gas flow from a micro wind tunnel attached to the polarizing microscope employed for optical measurements. Schlieren texture of the thin film consists of two stable domains separated by a domain wall. Gas or air flow on the liquid crystal surface induces director reorientation resulting in optical contrast. Transmitted and reflected light intensities from polarization microscopy provide measurement of the flow parameters. System response time τ∼150 μs has been estimated from the material viscosity and the time variation of the applied shear stress in the experiment. Optical response is linear for applied differential pressures up to ≊800 Torr beyond which it tends to saturate. Second-order nonlinear effects are observed for flow rates beyond ≊42 ℓ/min. The configuration used in the present method overcomes many of the limitations of similar measuring techniques including those using cholesteric liquid crystals. The present method offers a preferred alternative for flow visualization and skin friction measurements in wind-tunnel experiments on laminar boundary layer transition investigations.

Surface-stabilized ferroelectric liquid-crystal electro-optic waveguide switch

We demonstrate control of the optical coupling between a pair of planar waveguides by application of voltage to surface-stabilized ferroelectric liquid-crystal films between them. A prototype switch exhibited a response time of 200 μs and a 40:1 switching ratio.

Dynamical behavior of thin ferroelectric liquid-crystal films in ac electric fields.

We study both experimentally and theoretically the behavior of a freely suspended ferroelectric smectic-${\mathit{C}}^{\mathrm{*}}$ liquid-crystal film in the presence of an ac electric field. The field drives the system periodically between two ordered phases with opposite polarization. We find several different regimes of dynamical behavior depending on the strength of the field. These results are explained in terms of a phenomenological free energy.

Novel Method for Aligning Ferroelectric Liquid Crystal on Substrate

We have depeloped a novel method for transferring a well aligned ferroelectric liquid crystal (FLC) thin film onto a solid substrate. A free-standing FLC film has been successfully obtained on a solid substrate with this method. A homeotropic alignment in smectic phase has been confirmed with polarizing microscope and X-ray diffraction. We will report this novel method for transferring a free-standing film and discuss some applications of this film.

Surface effects and defect structures in ferroelectric liquid crystals

The influence of boundaries on the smectic structure dielectric properties and polarization reversal processes in thin films of ferroelectric liquid crystals are discussed.

Modulated phases in thin ferroelectric liquid-crystal films.

We discuss modulated phases that can occur in thin liquid-crystal films composed of tilted, chiral molecules. While the phase diagram depends on all orders of a Landau expansion we find that either a striped phase of parallel defect walls or a lattice of hexagonal unit cells containing disclinations and bounded by intersecting walls can occur. The striped phase can occur with either a positive or negative bend elastic constant depending on the underlying microscopic parameters. The transition into the modulated phase is, in general, continuous with infinite defect separation at the transition.

Freely Suspended Ferroelectric Liquid-Crystal Films: Absolute Measurements of Polarization, Elastic Constants, and Viscosities

The quenching by an electric field of molecular-orientation fluctuations in freely suspended ferroelectric smectic-$C$ films has been studied by light scattering, allowing absolute measurements of polarization, elastic constants, and viscosities. Evidence is presented to demonstrate surface contributions to Frank elasticity and to show that Frank elasticity is nonlocal.